
// Definition for a binary tree node.
function TreeNode(val, left, right) {
    this.val = (val===undefined ? 0 : val)
    this.left = (left===undefined ? null : left)
    this.right = (right===undefined ? null : right)
}
/**
 * @param {TreeNode} root
 * @return {number}
 */
var maxDepth = function(root) {
  if(root == null)
    return 0;
  else
    return Math.max(maxDepth(root.left) + 1, maxDepth(root.right) + 1);
};

let tree = [3,9,20,null,null,15,7];

